The inventors of the present application proposed, in their earlier Japanese Patent Application Laying-Open No. 62-123965, a current-limit system that can be used in voltage-type PWM (pulse-width modulation) control inverters. An example of the principal circuit of a PWM control inverter to which the current-limit system may be applied is shown in FIG. 22. In FIG. 22, E.sub.1 designates a dc (direct current) power supply, INV, an inverter, Tr.sub.1 to Tr.sub.6, transistors, D.sub.1 to D.sub.6, feedback diodes that have inverse-parallel connections across the transistors Tr.sub.1 to Tr.sub.6, and IM denotes a three-phase induction motor (which will be simply referred to "motor") that serves as a load.
The inverter INV provides the constant V/f (voltage/frequency) control in the known manner: the appropriate command value for the output voltage of the inverter INV is generated based on the frequency set value for the output voltage; this command value is compared with a carrier waveform so that a PWM signal may be produced; and this PWM signal enables the transistors Tr.sub.1 to Tr.sub.6 to provide the output voltage of the inverter INV that meets the requirements specified by the output voltage command value mentioned above.
The output current limit system for the inverter INV is provided with a current transformer or the like to measure the magnitude of currents iu, iv, and iw of respective phases (u, v, w), in order to determined that any of the currents exceed their specific limit value. If one of the phase currents iu, iv, and iw exceeds its positive limit value, the transistor in the upper arm for that phase is turned off, whereas if one of the currents exceeds its negative limit value, the transistor in the lower arm for the phase is turned off. This current-limit system will be described more specifically below.
First, it is assumed that respective phase currents are positive when they flow in the direction shown in FIG. 22, in the case where the motor IM is driven by the inverter INV. Then, it is also assumed that each pair of transistors Tr.sub.1 and Tr.sub.4, transistors Tr.sub.2 and Tr.sub.5, and transistors Tr.sub.3 and Tr.sub.6 in the upper and lower arms that forms a phase u, a phase v, and a phase w, respectively, has a switching pattern represented by (Su, Sv, Sw). It is noted that the corresponding transistors in the upper arm are in the ON state when Su, Sv and Sw assume the value of "1", respectively, whereas the corresponding transistors in the lower arm are in the ON state when Su, Sv and Sw assume the value of "0" respectively.
If the current of the phase u exceeds its positive limit value when the switching pattern is (1 0 0) (which means that transistors Tr.sub.1, Tr.sub.5 and Tr.sub.6 are in the ON state), the switching pattern for the phase u will be forced to change from "1" to "0". Changing the switching pattern to (0 0 0) causes the transistor Tr.sub.1 in the upper arm for the phase u to turn off, decreasing the u-phase current flow. Changing the switching pattern from "1" to "0" with regard to u-phase means to turn off the transistor Tr.sub.1 in the upper arm while turning on the transistor Tr.sub.4 in the lower arm. Turning the transistor Tr.sub.1 off when the current flows in the positive direction makes the current to flow through the transistors in the lower arms of the other phases and through the diode D.sub.4 in the lower arm of the unphase. Therefore, the fact that the transistor Tr.sub.4 is turned on has no significant meaning. Rather, turning off the transistor Tr.sub.1 of the upper arm makes the current flow decrease.
If the u-phase current flow exceeds its negative limit value, the transistor Tr.sub.4 is turned off, thus decreasing the current flow.
The rate of change of the current vector i flowing through the motor IM can be obtained from the output voltage vector V of the inverter INV, the counter electromotive force vector e of the motor IM, and the motor leakage inductance l, and is approximately expressed by the following equation. EQU l.multidot.di/dt=(V-e) (1)
The output voltage of the inverter INV shown in FIG. 22 can provide eight types of voltage vectors because there are eight (2.sup.3) switching patterns. Those switching patterns can be represented by the respective voltage vectors consisting of the voltage vectors V.sub.1 to V.sub.6 and the zero voltage vectors V.sub.O, V.sub.7, which are spaced by every .pi./3 (rad) as shown in FIG. 23. Respective phase currents iu, iv and iw shown in FIG. 22 are also shown in FIG. 23.
To produce a desired voltage vector V, the PWM inverter INV described above forms an equivalent voltage vector V having the desired magnitude and angle: the voltage vectors adjacent to the voltage vector V and the zero voltage vectors V.sub.0, V.sub.7 (when the voltage vector V is located as indicated in FIG. 23, for example, the voltage vectors V.sub.3, V.sub.4 are adjacent to the vector V) are selected from the it eight voltage vectors V.sub.0 to V.sub.7 in a time division manner within a specific time interval so as to produce the desired voltage vector V. Thus, the inverter INV may be controlled according to the switching patterns which correspond to the voltage vectors V.sub.3, V.sub.4, V.sub.0 and V.sub.7.
Referring now to the vector diagram in FIG. 24, let us consider the way the output current of the inverter is limited when the motor IM in FIG. 22 is driven by the inverter.
Let us suppose that the inverter selects the voltage vector V.sub.1 (switching pattern of (1 0 0)) in a time division manner so as to produce output voltage Va in FIG. 24 while driving the motor IM, and that the u-phase current iu exceeds its positive limit value during this phase. In this case, the voltage vector V.sub.0 corresponding to the switching pattern (0 0 0) in which Su is set to "0" is selected. As a result, the direction of di/dt will be that of (l.multidot.di/dt).sub.1, being determined by the above equation (1), the counter-electromotive forces el and V.sub.0. This will decrease the u-phase current iu.
On the other hand, let us suppose that the inverter INV selects the voltage vector V.sub.3 (the switching pattern (0 1 0)) in a time division manner to produce output voltage V.sub.b while braking the motor IM, and that the u-phase current iu exceeds its positive limit value at that time. In this case, the conventional system changes the u-phase Su of the switching pattern to "0". However, the Su in this pattern is originally "0", and so the switching pattern remains (0 1 0). In other words, this switching pattern continues to provide output voltage V.sub.3. The direction of di/dt, in this case, will be that of (l.multidot.di/dt).sub.2, being determined by the above equation (1), the counter electromotive force e.sub.2 and V.sub.3. This direction possesses the positive component of the u-phase current iu. This presents a problem that the current iu cannot be limited (or decreased) by the above method.